Of 113 patients over 20 years of age who underwent simultaneous replacement of dysfunctioning native tricuspid and mitral valves at The National Institutes of Health between 1963 and 1989, 17 patients (15%) had purely regurgitant mitral valves. All 17 also had severe tricuspid valve regurgitation. Of the various ideologies of the mitral regurgitation, 5 had mitral valve prolapse, 2 had associated hypertrophic cardiomyopathy, 2 had secundum type atrial defect, 6 had infective endocarditis which healed and 2 had the hypereosinophilic syndrome. .Rheumatic heart disease was the cause of mitral regurgitation in only 1 patient. The ideology of the mitral regurgitation was unclear in 1 patient. The operatively excised tricuspid valve was normal but the tricuspid valve annuli were dilated in all 17 patients. In summary, the type tricuspid valve dysfunction occurring in association with pure chronic mitral regurgitation is virtually always pure regurgitation.